The present invention relates to a double-sided substrate including a ceramic substrate and metal layers formed on surfaces on the opposite sides of the ceramic substrate. The present invention relates also to a semiconductor device including the double-sided substrate and a method for manufacturing the semiconductor device.
A semiconductor device is known which includes a double-sided substrate having mounted thereon a semiconductor element and a heat sink which is joined to the double-sided substrate for allowing heat generated by the semiconductor element to be released to the heat sink through the double-sided substrate. The double-sided substrate includes a ceramic substrate, a metal layer formed on one side of the double-sided substrate and forming a wiring layer and another metal layer formed on the other side of the double-sided substrate and forming a joining layer. The semiconductor device is formed in such a way that the semiconductor element is joined to the wiring layer of the double-layer substrate and the heat sink is joined to the joining layer.
For accomplishing a good joint between the ceramic substrate and the metal layer of such semiconductor device under heat cycle, Japanese Patent Application Publication 2009-65144 discloses a semiconductor device including a joining layer made of laminated aluminum layers whose aluminum purities differ from each other.
More particularly, in the joining layer of the above Publication, the aluminum layer joined to the ceramic substrate is made of aluminum whose purity is 99.99 wt % or more (or 4N—Al) and the other aluminum layer joined to the heat sink is made of aluminum whose purity is 99.5 wt % or more, but less than 99.9 wt % (or 2N—Al). The hardness (or the strength) of 4N—Al is lower than that of 2N—Al and, therefore, the aluminum layer joined to the ceramic substrate tends to be warped easily due to heat stress generated under the heat cycle and may be separated from the ceramic substrate.
The present invention is directed to providing a double-sided substrate which has improved joint strength between a ceramic substrate and a metal layer, a semiconductor device including such double-sided substrate and a manufacturing method for the semiconductor device.